﻿100 
  ProP. 
  L. 
  11. 
  Ingersoll 
  on 
  Magnetic 
  

  

  half 
  shade 
  polariineter, 
  using 
  sodium 
  light. 
  An 
  opaque 
  film 
  

   was 
  deposited 
  on 
  plate 
  glass 
  and 
  the 
  Kerr 
  rotation 
  measured 
  

   with, 
  and 
  without, 
  the 
  use 
  of 
  Canada 
  balsam. 
  The 
  obser- 
  

   vations 
  were 
  very 
  difficult 
  and 
  not 
  very 
  satisfactory 
  in 
  the 
  

   latter 
  case, 
  but, 
  when 
  corrected 
  for 
  the 
  doubled 
  rotation 
  of 
  

   the 
  glass 
  cover 
  slip, 
  came 
  out 
  about 
  half 
  larger. 
  

  

  SUMMAEY. 
  

  

  1. 
  The 
  writer's 
  bolometric 
  method 
  of 
  measuring 
  magnetic 
  

   rotations 
  has 
  been 
  developed 
  until, 
  under 
  favourable 
  circum- 
  

   stances, 
  rotations 
  of 
  less 
  than 
  001° 
  can 
  be 
  detected, 
  and 
  

   angles 
  somewhat 
  larger 
  measured 
  with 
  an 
  accuracy 
  approxi- 
  

   mating 
  1 
  per 
  cent. 
  It 
  is 
  applicable 
  to 
  a 
  range 
  of 
  spectrum 
  

   from 
  the 
  D 
  lines 
  to 
  X 
  = 
  2'2yLt. 
  

  

  2. 
  The 
  dispersion 
  of 
  magnetic 
  rotation 
  for 
  cases 
  of 
  reflexion 
  

   from 
  and 
  transmission 
  tli 
  rough, 
  iron 
  cathode 
  films, 
  has 
  been 
  

   measured. 
  For 
  the 
  pure 
  iron 
  films 
  these 
  curves 
  are 
  entirely 
  

   similar 
  and 
  show 
  a 
  gradual 
  increase 
  of 
  rotating 
  power 
  until 
  

   wave-length 
  1'5/x 
  is 
  reached, 
  where 
  the 
  rotation 
  reaches 
  a 
  

   maximum 
  value 
  over 
  one 
  and 
  two-thirds 
  times 
  as 
  great 
  as 
  

   that 
  for 
  the 
  D 
  lines. 
  

  

  3. 
  After 
  a 
  certain 
  thickness 
  is 
  reached 
  the 
  reflexion 
  rotation 
  

   ceases 
  to 
  increase 
  further 
  with 
  thickness 
  of 
  film. 
  A 
  layer 
  of 
  

   iron, 
  of 
  thickness 
  less 
  than 
  20/u/z., 
  or 
  about 
  that 
  of 
  the 
  assumed 
  

   " 
  transition 
  layer 
  " 
  of 
  optical 
  theory, 
  gives 
  rise 
  to 
  considerably 
  

   more 
  than 
  half 
  of 
  this 
  rotation. 
  

  

  4. 
  Partially 
  oxidized 
  films 
  do 
  not 
  show 
  the 
  similarity 
  

   between 
  the 
  reflexion 
  and 
  transmission 
  rotation 
  dispersion 
  

   curves 
  that 
  pure 
  films 
  do. 
  The 
  rotation 
  for 
  oxidized 
  films 
  

   reaches 
  a 
  maximum 
  in 
  the 
  early 
  infra-red 
  and 
  falls 
  off 
  rapidly 
  

   Avith 
  further 
  increase 
  in 
  wave-length. 
  This 
  change 
  in 
  form 
  

   of 
  the 
  rotation 
  curve 
  with 
  oxidation 
  is 
  accompanied 
  by 
  a 
  

   corresponding 
  change 
  in 
  the 
  transmitting 
  or 
  reflecting 
  powers 
  

   as 
  a 
  function 
  of 
  wave-length. 
  

  

  5. 
  The 
  Kerr 
  rotation 
  is 
  nearly 
  proportional 
  to 
  the 
  refractive, 
  

   index 
  of 
  the 
  overlyirig 
  medium, 
  which, 
  with 
  certain 
  assump- 
  

   tions, 
  may 
  be 
  shown 
  to 
  be 
  in 
  accordance 
  with 
  Voigt's 
  

   theory. 
  

  

  Physical 
  Laboratory, 
  

  

  University 
  of 
  Wisconsin, 
  

  

  Oct. 
  31, 
  1908. 
  

  

  Addendum. 
  

  

  Since 
  the 
  above 
  was 
  written, 
  Skinner 
  and 
  Tool's 
  paper 
  on 
  

   the 
  optical 
  properties 
  of 
  magnetic 
  films 
  has 
  appeared 
  (Phil. 
  

   Mag. 
  Dec. 
  1908). 
  It 
  is 
  a 
  valuable 
  contribution 
  to 
  the 
  

   literature 
  of 
  the 
  subject, 
  showing 
  very 
  clearly 
  as 
  it 
  does 
  the 
  

  

  